1. Field of the Invention
The present invention relates to a pneumatic safety tire and especially, the invention provides an improved pneumatic safety tire which can be used safely without there happening any separation of the bead portions of the tire from the rim flange (hereafter, this accident will be called as bead/rim separation) during running even in the so called "run flat" state which occurs due to an extreme decrease of the inner pressure within the tire or to a puncture (hereafter, the state will be called "run flat",) and moreover, it has a sufficient ability to bear a load and a sufficient durability.
2. Description of the Prior Art
As the conditions, necessary for a pneumatic safety tire, the following items can be pointed out. That is, the conditions are (A) bead/rim separation does not happen even when a puncture of the tire happens, (B) even when a puncture of the tire does happen, it should be possible to continue driving without rapidly changing the vehicle's running direction owing to the puncture shock and to brake it to a stop after a short period of normal running and (C) even in case of "run flat", the tire should have a sufficient durability and offer a sufficient safety in driving to permit the car to be run at an appropriate running speed for a necessary distance, in order to make it possible to safely escape from the highway by maintaining a not extremely bad driving condition.
In other words, it can be concluded that the necessary conditions for a pneumatic safety tire are that even in the case of a flat tire, bead/rim separation does not occur and moreover, the cornering power and cornering force do not extremely decrease.
Pneumatic safety tires for cars, tires which not only have an ability to run just the same as general tires in the normal state where the inner pressure is normal, but also have an ability to run to some extent even in a punctured state, have already been proposed. Examples of those tires are (1) a tire whose air chamber is filled with foam, (2) a tire provided with a supporter within it, in order to support the load in an emergency case of "run flat", (3) a tire having a double chamber structure, by inserting another tire within it, (4) a tire provided with a ring-formed projection on the outside of the rolling axis along the rim flange and (5) a tire whose side wall and/or shoulder are reinforced.
Those tires exemplified above have following disadvantages. The tire exemplified as (1) is not acceptable for use on an ordinary car, since it generates a lot of heat by high speed running. In case of the tire (2), though it is possible to satisfy the item (A) of the necessary conditions mentioned above, in order to satisfy the items (B) and (C), it necessitates providing a strong supporter of a sufficient height and moreover to use a lubricating agent, etc. in order to decrease the frictional resistance between the supporter and the inner portion of the tire. This brings about a considerable increase of the weight and increase of the complexity in mounting and dismounting the tire, which is a great disadvantage of a tire of this type. The tire exemplified as (3) still has the possibility that a puncture of both inside tire and outside tire can happen at once and also it has the same disadvantage as the tire, (2) such as the necessity to use a lubricating agent, the increase of complexity in mounting it onto a rim and dismounting it from the rim and the increase of the weight of the tire itself as mentioned above. As an example of the tire classified as (4), U.S. Pat. No. 2,037,640 can be exemplified. This type of tire is surely capable of satisfying the item (A) of the necessary conditions, however, the production of the tire (4) and its mounting and dismounting are very difficult and also the items (B) and (C) of the necessary conditions can never be satisfied and, moreover, it should be pointed out that the tire belonging to the class (4) is not proper for use on any ordinary car which runs at a high speed. The tire disclosed in U.S. Pat. No. 2,874,745 relates to a tire which can be applied for agricultural use and even though it is possible to satisfy the item (A) of the necessary conditions, it can never satisfy the items (B) and (C) and, moreover, its mounting and dismounting are very difficult to carry out. Further, the tire disclosed in U.S. Pat. No. 4,061,172 satisfies the item (A) of the necessary conditions and those tires disclosed in U.S. Pat. No. 3,983,920 and U.S. Pat. No. 4,034,792 can satisfy the item (A) only in a case of very slow turning speed. However, since the side portions of those tires cannot bear the load in the state of "run flat", a considerable decrease of cornering power is inevitable. Thus, in those tires, the running speed and the possible distance to be driven in an emergency case of "run flat" extremely decrease and accordingly, it is concluded that the durability and the stability in driving are almost lost.
Thus, the tire described in (5) has been spot-lighted as a useful pneumatic tire, and it has been called a Combat type tire. However, a tire of this type has the following defects and accordingly, it leaves problems to be solved. Each tire is disclosed in U.S. Pat. No. 3,911,987, U.S. Pat. No. 3,949,798, U.S. Pat. No. 3,954,131, U.S. Pat. No. 3,964,532 and U.S. Pat. No. 3,983,919, which belong to the classification (5), have such a structure that an elastic reinforcement is inserted between a pair of carcass plies or is added within the inside of the tire at the region corresponding to the portion from the side wall to the shoulder (hereafter, the portion will be called as the side portion). However, an easy occurrence of bead/rim separation during "run flat" is the first defect of the tire of this type.
It is impossible in the tire of this type to satisfy the items (A), (B) and (C) of the necessary conditions mentioned above, unless a rim of the flat base type is used, or another element to prevent the bead/rim separaton is used together with the tire in order to remove said defect. Furthermore, it should be pointed out that even if said defect is removed by such a counter measure as mentioned above, the appearance of other defects such as cracking in the inside of the shoulder in normal running and the separation of the shoulder portion due to an excessive heat build-up in high speed running, etc. are inevitable, since it is necessary to make the thickness of the side portion considerably large in order to bear the load in the emergency case of "run flat". Furthermore, it is true that if a conventional rubber compound is used for the material to reinforce the side portion, decomposition and carbonization of the rubber layer occur due to a violent generation of heat in the running during "run flat" and with the progress of such phenomena, a flex fatigue abrasion occurs and finally, the structure of the side portion is destroyed and further running of the tire becomes impossible.